Various types of ADPCM speech compression systems are in use today throughout the world. The relevant recommendation G. 721 of the International Telegraph and Telephone Consultative Committee (CCITT) appearing in Volume III, Fascicle III.3 of the Red Book issued at the Malaga-Torremolinos Plenary Assemby of 19 Oct. 1984 indicates at Appendix I that the ADPCM systems which meet the present CCITT standard do not provide satisfactory voiceband data transmission at 9600 bit/s.
In Report R 17 of Study Group XVIII-of the CCITT dated April, 1986, four general directions were proposed for enabling ADPCM apparatus to provide satisfactory voiceband data transmission at 9600 bit/s:
A. Provision of a new ADPCM algorithm at 32 Kbit/second without voiceband data discrimination;
B. Use of 40 Kbit/s ADPCM based on Recommendation G.721 with a data rate analyzer for voiceband data discrimination;
C. Use of a data-optimized ADPCM algorithm at 32 Kbit/s and detecting the V. 29 training signal to provide voiceband data discrimination; and
D. Use of a data optimized algorithm at 32 Kbit/s and detecting the header tone to provide voiceband data discrimination.
Proposal D was originated by the present applicant, but the technique and apparatus for carrying it out was not disclosed.